The ability of micropumps to generate high-pressures is appealing because it enables integration of a complete high performance liquid chromatographic (HPLC) system on a lab-on-a-chip (LOC) platform. HPLC is arguably the most commonly utilized separation technique for chemical analysis. HPLC can analyze 80% of all known compounds and it is widely used from drug compound screening to medicine production, from clinical diagnosis to food quality examination, and from environmental protection to space exploration, for example. Miniaturizing a HPLC system is of great significance because these systems also allow samples to be analyzed at the point of need rather than a centralized laboratory. LOC devices provide a new class of research tools for the investigation of life processes and drug discovery. Currently, few existing micropumps can produce adequate pumping pressure for LOC HPLC applications.
Numerous micropumps have been developed. The electroosmotic pump (EOP) has emerged as one of the most promising candidates that have shown potential for practical HPLC separations. EOPs have several inherent advantages over other types of micropumps. For example, EOPs are capable of generating pulse-free flows, EOP flow magnitude and direction are convenient to control, EOPs can be fabricated using standard microfabrication technologies (and thus are readily integratable with LOC devices), and EOPs have no moving parts.
The first EOP was developed in early 1970's when a 5-cm-long and 1-mm-inner diameter (i.d.) glass column was packed with 1-20 μm silica particles to create electroosmotic flow (EOF) to drive HPLC separations. The working pressure of this pump was about 40 bars. A capillary packed with 1-3 μm silica beads was used to generate EOF, and the maximum pressure created was about 350 bars. Parallel columns packed with 2-3 μm silica beads produced a maximum pressure of about 150 bars. Polymer and silica-based monolithic and open capillary EOP's have also been developed, but pressures produced by these pumps were rarely over 100 bars.
Another previously developed configuration of EOP relies on open capillaries and includes a first EOP pump unit made from open capillaries coated with a positively-charged polymer, and a second EOP pump unit made from open capillaries coated with a negatively-charged polymer. The pump units can be joined or coupled in series to enhance pumping pressures, and a maximum pressure of about 200 bars has been achieved with such open capillary EOPs. However, due to the typical pressures used in HPLC separations, multiple open capillaries have to be utilized in parallel to produce adequate flow to drive HPLC separations.